1. Safety first: Before you disturb any painted surface, you should think about the possibility of lead. Lead paint additives were phased out in the 1970's and 80's. If you suspect lead is present in any paint, you can find information about safety precautions and testing at www.epa.gov/lead, or by calling the National Lead Information Center at 1-800-424-LEAD. Even with newer surfaces, I find that a dust mask is always in order when sanding. Your lungs will thank you.
2. Sanding: Not all pieces will require sanding before repainting. My mantra is to leave well enough alone when dealing with old paint surfaces. If, however, the paint is chipped, peeling, crackling, or has other problems, you might not be able to avoid the sandpaper. A small electric hand sander is my best friend for these situations. A very light sanding (really just scuffing) can help your new paint adhere to old oil paint or a glossy surface.
3. Prep, prep, prep: The number one reason for paint failures is incorrect preparation. Time spent on prep will ensure you get a good result in the end. Make sure the surface is completely clean and dust free before you ever get the paint out of the can. If you have sanded the surface, remove dust with a tack cloth. This is a small piece of cheesecloth soaked in wax, available at your local home improvement store in the paint section. They cost around one dollar...get several to keep on hand for the next project! If you can't find one, several thorough passes with a damp cloth followed by another pass with a dusting cloth can do the job. Most importantly, don't rush your prep work.
4. Taping: Carefully tape off any areas you don't want painted, and burnish the tape edges down once, firmly, with a fingernail. If you think you might have to leave the tape on for more than a day or so, use a lower adhesion tape made for delicate surfaces.
5. Priming: If the piece is unpainted, consider a coat of primer before you paint. There are excellent primers specifically made for stone and metal. Tell your paint store rep what you're painting and follow her advice. Use a good latex or oil-based primer for wood surfaces. Some wood grains (pine or cheaper grades of oak, for example) may 'rise' after a first coat of latex, and priming improves the chances that you can avoid this. The raised grain will feel rough. If this happens, you'll need to sand the grain down lightly and prime again. Don't worry; it's not as bad as it sounds when you know what happened!
6. Painting: Finally, time to paint! Use a good quality paint brush. Be careful not to overload your brush with paint and put it on too thickly. This is a common mistake. Painting in thick layers can lead to ugly drips, cracks, or drying delays. Instead, paint in thin coats, allowing time for each coat to dry completely before recoating. This will give you a beautiful and strong surface.
7. Curing: Follow the instructions on your paint can to make sure your new furniture is safely cured before you begin to use it or place objects on top of it. Paints, latex in particular, can feel dry well before they really are. When in doubt, I like to wait 24-48 hours before I place my newly painted flea-market beauties into service.
Now, enjoy your new furnishings. Use them well and never fear; if they get chipped or you tire of the color, you know how to fix it!